During the manufacture of integrated circuits (ICs), and especially in the process of wafer polishing for very large scale integration (VLSI) applications--either as the last step in wafer shaping or, further downstream, in chemo-mechanical polishing (CMP) for layer planarization--the achievement and/or maintenance of a high degree of wafer planarity is of utmost importance.
The achievement of optimal wafer flatness, both in the initial forming of the wafer and after initial forming during CMP, directly impacts yield and overall manufacturing costs, since wafer flatness is fundamental to achieving desired focal conditions during the various lithographic printing steps applied during IC manufacture. This is especially true for advanced, e.g. 0.35 .mu.m or less, technologies. The high yields required for today's competitive IC manufacturing can only be achieved if optimal focal conditions over the whole wafer are obtained.
Success in achieving or maintaining planarity depends on a variety of conditions such, for example, as initial wafer flatness from lapping and etching, the polishing process itself, the type of machine utilized in the process, and many other machine and operator related variables. Also affecting the IC end product are the consistency, type and uniformity of the consumables employed in the polishing process.
Among the consumables employed during wafer processing are the polishing pad, the slurry and, most importantly, the soft polymeric mounting film that the wafer is attached to and held by, usually via capillary attraction or friction, during the polishing process. Among the most critical parameters of the process are the flatness and shape of this mounting film.
Depending upon the process employed, the mounting film may be unattached or laminated to other structures, usually to a plastic film material such as "MYLAR" or fiberglass-epoxy based materials. The film may have a high, medium or low compressibility and wafer-holding ability, depending upon its pore size, density, thickness, surface condition or other treatments applied. In several commonly known forms the mounting film may have a pressure sensitive adhesive backing for ease of use and quick attachment to a wafer carrier. The general properties and use of such films are known, and described elsewhere, for example in U.S. Pat. No. 4,512,113 to Bodinger.
In IC manufacturing applications utilizing mounting films of this type, if the film is not uniformly compressible, is overly or insufficiently compressible, is not sufficiently flat, is incomplete, or is otherwise defective, the ultimately-required wafer flatness--or layer uniformity in CMP planarization--cannot be achieved. Ideally, in order to achieve a desired wafer flatness or layer uniformity the flatness of a mounting film assembly should be equal to or better than the required wafer flatness. The present invention flows from a recognition that the flatness of mounting films in current use often does not approach that ideal condition.
Presently, the flatness characteristics of mounting films--sometimes referred to in the industry as qualifications--are most often obtained from information provided by the manufacturer, since the films are typically sold in batches on the basis of a single thickness measurement taken at a representative point on the film; this is commonly known as thickness sorting. Such thickness sorting exhibits a significant drawback in that it does not take into account thickness variations across the entire surface of interest of the mounting film, and neither does it address thickness variations imposed by the mounting film assembly that is used as an associated part of the overall wafer preparation process.
Another conventionally-practiced method is to obtain the required flatness information subjectively after the wafer has been processed--i.e. after polishing--by testing the wafer flatness (or the layer uniformity in CMP). This measured result is then related to the actual conditions of the mounting film. Unfortunately, this measurement is most often arrived at after numerous wafers have already been polished and rejected. Another disadvantage of this technique is that it does not permit one to determine whether, or to what extent, variables other than the flatness of the mounting film are responsible for the substandard wafer surface.
It has become increasingly apparent that low yields in the flatness of certain wafer lots can be and often are directly related to variations in the surface characteristics of the mounting films used in wafer processing. Yet to date there exists no satisfactory way to obtain critical surface measurement of wafer-carrying mounting films to determine whether the film meets preselected, application specific flatness criteria . Currently known mechanical surface measuring techniques are unable to measure the entire surface of the subject film with sufficient precision due at least in part to the softness of the film itself. Moreover, direct optical measurement of mounting films of interest using interferometry is currently impossible due to the almost total non-reflectivity of these mounting films.
There is accordingly an urgent and critical need for a method of testing the flatness and other critical surface characteristics of the mounting films used in wafer processing and IC manufacture prior to actual production of the wafer end product so as to increase the yield of usable wafers, and thereby improve productivity, by identifying deficient mounting films before they are employed in the wafer production process.
As applied to IC manufacture, the above-referenced ability to cull mounting films in accordance with design-dependant, pre-selected parameters yields superior results during initial wafer forming and chemo-mechanical polishing, resulting in an improvement in the ability to meet critical IC design tolerances, especially in processes requiring design parameters of 0.35 .mu.m or less. Integrated circuit devices thus obtained may be, for example, microprocessors, logic chip arrays, CMOS DRAMs, and other single or multilayer products.
Such a method would also be of great value to film suppliers as a tool for enhanced thickness sorting and qualification, as well as for measuring improvements in the manufacture of these films.